Scanning radar

ABSTRACT

A scanning radar system accurately determines the lateral position of a target regardless of the heading of the target. The angle Ψ between the direction to the target and the heading of the target is calculated using equation  
     Ψ=θ−tan −1    {d  cos θ/( R−d  sin θ)} 
     and a correction value ΔX for the lateral position X is determined based on the angle Ψ.

TECHNICAL FIELD

[0001] The present invention relates to a scanning radar system thattransmits radio waves while scanning the projection direction thereof,that determines the lateral position of a target based on the strengthof a reflected wave returned from the target and, more particularly, toa scanning radar system that measures the lateral position of a targetas well as the distance and relative velocity thereof forvehicle-to-vehicle distance control.

BACKGROUND ART

[0002] A vehicle equipped with an FM-CW radar can measure the distanceand relative velocity of a target located ahead of the vehicle. If thedistance, for example, to a vehicle traveling ahead is to be properlycontrolled based on these measured values, the lateral position of thetarget located ahead must be determined.

[0003] A scanning radar system, as shown in FIG. 1(b), transmits aradiowave while scanning the projection direction thereof, anddetermines the lateral position of a target based on the direction inwhich the target is located, i.e., the direction in which the strengthof the reflected wave from the target is the highest.

[0004] According to this method, the lateral position of the target canbe determined accurately, provided that there is no displacement betweenthe direction to which the target is located when viewed from theradar-equipped vehicle (the direction to the target) and the directionin which the target is traveling (the heading of the target).

[0005] However, if there is a displacement between the direction to thetarget and the heading of the target, the lateral position of the targetcannot be determined accurately. For example, in the case shown in FIG.1(a) or 1(c), as the power of the reflected wave is the highest at oneedge of the target, the lateral position of the target cannot beestimated accurately.

DISCLOSURE OF THE INVENTION

[0006] Accordingly, it is an object of the present invention to providea scanning radar system that can accurately determine the lateralposition of a target even when the heading of the target is displacedfrom the direction to the target.

[0007] According to the present invention, there is provided a scanningradar system comprising: lateral position determining means fordetermining a lateral position X of a target, based on the strength of areflected wave returned from the target when radio waves are projectedwhile scanning the projection direction thereof; means for determiningan angle Ψ between a direction to the target and a heading of thetarget; means for determining a correction value ΔX for the lateralposition based on the angle Ψ; and means for correcting the lateralposition X by the correction value ΔX.

[0008] Preferably, the angle Ψ determining means determines the angle Ψ,based on a turning radius R, a distance d to the target, and an angle θbetween the direction to the target and the heading of a vehicleequipped with the radar system, by calculating equation

Ψ=θ−tan⁻¹ {d cos θ/(R−d sin θ)}

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a diagram for explaining a problem in determining thelateral position;

[0010]FIG. 2 is a block diagram showing the configuration of avehicle-mounted radar system according to one embodiment of the presentinvention;

[0011]FIG. 3 is a diagram for explaining a method of calculating theangle Ψ;

[0012]FIG. 4 is a diagram for explaining a method of calculating theangle Ψ;

[0013]FIG. 5 is a diagram for explaining how the correction value ΔX iscalculated from the angle Ψ;

[0014]FIG. 6 is a diagram showing a second example explaining how thecorrection value ΔX is calculated from the angle Ψ;

[0015]FIG. 7 is a diagram showing a third example explaining how thecorrection value ΔX is calculated from the angle Ψ;

[0016]FIG. 8 is a diagram showing a first example explaining how ΔX iscorrected according to the distance d or turning radius R; and

[0017]FIG. 9 is a diagram showing a second example explaining how ΔX iscorrected according to the distance d or turning radius R.

BEST MODE FOR CARRYING OUT THE INVENTION

[0018]FIG. 2 shows the configuration of a vehicle-mountedmillimeter-wave scanning radar system as one embodiment of a scanningradar system according to the present invention.

[0019] In FIG. 2, an ECU 10 calculates the turning radius R of theradar-equipped vehicle based on a signal from a yaw rate sensor 12 and asignal from a vehicle speed sensor 14, and supplies the result to anFM-CW radar 16 together with vehicle speed data. The turning radius Rcan also be calculated by using data from a steering sensor instead ofthe data from the yaw rate sensor 12. The FM-CW radar 16 projects radiowaves, in the millimeter wave region and frequency modulated by atriangular wave, in the forward direction of the vehicle and calculatesthe distance and relative velocity of a target located ahead. Further,the FM-CW radar 16 scans the projection direction of the radio waves, asearlier described, and calculates an estimated value X[m] for thelateral position of the target from the power distribution of thereflected wave. It also calculates a correction value ΔX for the lateralposition based on the data of the turning radius R, etc. supplied fromthe ECU 10, and supplies the corrected lateral position X to the ECU 10together with the distance and relative velocity data. Based on thesedata, the ECU 10 generates and outputs a control signal for maintaininga constant distance from the vehicle traveling ahead.

[0020]FIGS. 3 and 4 show the principles of methods for calculating thelateral position X and the correction value ΔX. In FIGS. 3 and 4, R isthe turning radius of the radar-equipped vehicle 20, d is the distanceto the target 22, Ψ is the angle between the direction to which thetarget 22 is located when viewed from the vehicle 20 (the direction tothe target) and the direction in which the target 22 is traveling (theheading of the target), and φ is the angle between the heading of thevehicle 20 and the heading of the target 22.

[0021] As can be seen from FIG. 4, the angle φ can be calculated as

φ=tan⁻¹ {d cos θ/(R−d sin θ)}

[0022] Hence, the angle Ψ is given as

Ψ=θ−φ

[0023] Since the angle Ψ represents the displacement between thedirection of the radiowave projected to the target and the heading ofthe target, the correction value ΔX is determined in accordance with alinear function passing through the origin, such as shown in FIG. 5.Based on the thus determined ΔX, the lateral position X is corrected as

X=X+ΔX

[0024] These operations are performed using software, for example, byincorporating a CPU in the FM-CW radar.

[0025] In the function shown in FIG. 5, ΔX increases with increasing Ψ,but it is desirable that an upper bound be placed on the absolute valueof ΔX as shown in FIG. 6, for reasons such as an actual vehicle widthbeing finite. It is further desirable that a dead zone where ΔX ismaintained at 0 be provided centered about angle 0 as shown in FIG. 7,because it is not desirable for the lateral position X to vary for smallvalues of Ψ due to the effect of noise. Further, when the distance tothe target is large, the correction amount ΔX should become small;therefore, ΔX should be reduced in order to avoid the effect of noise.In this case, ΔX is multiplied by a correction coefficient that becomessmaller than 1.0 when the distance d increases, as shown by the functionof FIG. 8 or 9, and the result is taken as ΔX. Likewise, when theturning radius R is large, the correction coefficient is reduced inorder to avoid the effect of noise.

[0026] Since the thus calculated ΔX contains noise, it is desirable toreduce the effect of noise by correcting X by using, for example, ΔX_(n)calculated as

ΔX _(n)=(ΔX _(n−1)×3+ΔX)/4

[0027] rather than directly correcting X by using ΔX. Further, when itis judged, from the uncorrected lateral position X, that the target istraveling in the same lane as the radar-equipped vehicle, the amount ofcorrection is reduced by multiplying ΔX, for example, by 0.7. When theamount of change of the turning radius R within a unit time becomesgreater than a predetermined value as a result of the steering action ofthe driver, the correction amount ΔX is set to 0 because it is notdesirable to correct X by responding to the change. No correction isapplied to the lateral position for a target judged to be a stationaryobject. Further, learning of a neutral position is performed in order tocancel the drift of the steering sensor (or the yaw rate sensor) basedon which the turning radius R is calculated; here, correction of thelateral position X should not be performed until the learning is done.

[0028] The calculation of the correction value ΔX has been describedabove, but it will be understood that, depending on the target, it isdesirable not to correct the lateral position if it is found for someother reason that correction of the lateral position is not necessary,even when the calculated ΔX value is not 0.

[0029] More specifically, for a target that matches any one of thefollowing conditions, no correction is applied because the correction isconsidered unnecessary even if the calculated ΔX value is not 0.

[0030] (1) A target that is judged, from the uncorrected lateralposition X, to be traveling in the same lane as the radar-equippedvehicle.

[0031] (2) A target from which the power (peak strength) is smaller thana predetermined fixed reference value or a reference value that variesas a function of the distance. The reason is that, in the case of asmall target such as a bicycle, the reflected wave (peak) is less likelyto be distributed in the angular direction.

[0032] (3) A target from which the reflected wave (peak) is notdistributed in the angular direction but exhibits a peak in only onedirection.

[0033] (4) A target that has been judged to be an object, such as alarge truck, physically having a plurality of reflecting points, and forwhich processing has been performed to take the average over a pluralityof reflected waves (peaks) giving similar distances, angles, andrelative velocities. The reason is that the angle is already correctedby the averaging operation and, if a further correction were applied, anovercorrection would result.

[0034] For any target for which a correction has ever been made bydetermining that it does not match any one of the above conditions (1)to (4), it is desirable to always treat such a target as a target thatcauses an angular displacement and apply a correction, even if thetarget is thereafter judged to match any one of the conditions (1) to4).

[0035] As described above, according to the present invention, there isprovided a scanning radar system that can accurately determine thelateral position of a target regardless of the heading of the target.

1. A scanning radar system comprising: lateral position determiningmeans for determining a lateral position X of a target, based on thestrength of a reflected wave returned from said target when a radiowavewas projected while scanning the projection direction thereof; means fordetermining an angle Ψ between a direction to said target and a headingof said target; means for determining a correction value ΔX for saidlateral position based on said angle Ψ; and means for correcting saidlateral position X by said correction value ΔX.
 2. A scanning radarsystem according to claim 1, wherein said angle Ψ determining meansdetermines said angle Ψ, based on a turning radius R, a distance d tosaid target, and an angle θ between the direction to said target and theheading of a vehicle equipped with said radar system, by resolving theequation Ψ=θ−tan⁻¹ {d cos θ/(R−d sin θ)}
 3. A scanning radar systemaccording to claim 1 or 2, wherein said correction value determiningmeans determines said correction value ΔX in such a manner that an upperbound value and a lower bound value are provided for said correctionvalue ΔX.
 4. A scanning radar system according to any one of claims 1 to3, wherein said correction value determining means determines saidcorrection value ΔX in such a manner that said correction value ΔX has adead zone where said correction value ΔX does not change even if saidangle Ψ changes within a range including
 0. 5. A scanning radar systemaccording to any one of claims 1 to 4 wherein, when the distance d tosaid target is large, said correction value determining means takes asmaller value as said correction value ΔX than when the distance to saidtarget is small.
 6. A scanning radar system according to any one ofclaims 1 to 5 wherein, when said turning radius R is large, saidcorrection value determining means takes a smaller value as saidcorrection value ΔX than when said turning radius is small.
 7. Ascanning radar system according to any one of claims 1 to 6 wherein,when said target is judged to be traveling in the same lane as theradar-equipped vehicle, said correction value determining means takes asmaller value as said correction value ΔX than said determined value ofΔX.
 8. A scanning radar system according to any one of claims 1 to 7wherein, when the amount of change of said turning radius R within aunit time is greater than a predetermined value, said correction valuedetermining means sets said correction value ΔX to
 0. 9. A scanningradar system according to any one of claims 1 to 8, further comprisingtarget discriminating means for determining, for each individual target,whether correction of said lateral position is necessary or not, andwherein, for any target for which said target discriminating means hasdetermined that correction of said lateral position is not necessary,said lateral position correcting means does not correct said lateralposition regardless of said determined correction value ΔX.
 10. Ascanning radar system according to claim 9 wherein, for any target thathas ever been determined as needing a correction to said lateralposition, said lateral position correcting means always correct saidlateral position.
 11. A scanning radar system according to claim 9 or 10wherein, for a target that is judged, from the uncorrected lateralposition X, to be traveling in the same lane as the radar-equippedvehicle, said target discriminating means determines that correction ofsaid lateral position is not necessary.
 12. A scanning radar systemaccording to claim 9 to 11 wherein, for a target that is judged tosubstantially not spread in an angular direction, said targetdiscriminating means determines that correction of said lateral positionis not necessary.
 13. A scanning radar system according to claim 9 to 12wherein, for a target for which an average angle has been taken over aplurality of reflections, said target discriminating means determinesthat correction of said lateral position is not necessary.